While numerous methods are available for synchronizing cells in culture, no adequate method has yet been described for synchronizing cells in vivo, i.e., in the intact animal. Several methods have been described in which some sort of parasynchronization can be achieved but no true effective synchronization of cells in the intact animal has yet been reported. Yet, this is an area of primary importance and of potential practical application, worthy of consideration. Since it has been demonstrated that cells differ in their sensitivity to X- rays or drugs in various phases of the cell cycle, it would be highly desirable to synchronize cells in vivo, so that they can be exposed to the therapeutic agent at the moment in the cell cycle in which they are most exquisitely sensitive. If the therapeutic agent can be directed, as for instance an X-ray beam, then a maximized result could be achieved in which these cells in the cycle can be destroyed, while the differentiated, normal cells surrounding the tumors are not damaged. Preliminary experiments in our laboratory have shown that methotrexate in certain combinations of doses and timing, together with other drugs, can give fairly reasonable degrees of synchronization. Studies ought to be continued along these lines until a good degree of synchronization can be obtained with a proper combination of drugs and a proper time schedule. The mucosa of the colon ought to be selected for these studies, since the cell cycle of cells in the intact animal varies greatly and different time schedules may have to be followed to obtain the best level of synchronization in different tissues.